This invention relates to the copolymers of 2,5-dihydrofurans and maleic anhydride, and heat curable epoxy resins comprising the copolymers.
Maleic anhydride polymers are commerically used in a wide variety of applications such as coatings. adhesives, thickeners, detergent builders, epoxy resins and the like. For many of these applications, such as for epoxy resins, it is desirable to use polymers having particular solubilities and melting points. Such properties are affected by the mol ratio of components in the repeating monomer of the polymer. For example, it is generally known that maleic anhydride copolymerizes to form alternating copolymers, i.e., each monomer unit of the polymer contains an average of one mol of maleic anhydride per mol of alternative component. Accordingly, while no absolute rules can be used to predict the physical properties of maleic anhydride alternating copolymers, it has been found the alternating copolymers are not suited for certain applications due to their melting point or solubility characteristics. For example, maleic anhydride has been copolymerized with 2,3-dihydrofuran to form a water-insoluble alternating copolymer having a reported melting point of approximately 200.degree. C; see for example Bunzo et al, Kogyo Kagaru Zasshi, 1970, 73(3), 580-6. Other cyclic ethers, such as epichlorohydrin, have been copolymerized with 2,5-dihydrofuran to prepare linear alternating copolymers by a ring-opening mechanism; see for example Yuji et al, Makromol Chem., 1968, 119 (Dec.) 104-12. While these copolymers are satisfactory for many applications, it has been found that they are ineffective as epoxy resin curing agents in heat curable compositions.
It remains desirable to provide a maleic anhydride/dihydrofuran copolymer having a melting point substantially below 200.degree. C which retains the ring structures of its components and is useful as an epoxy resin curing agent.